The disclosed invention is directed generally to computer data storage digital audio tape (DAT) drives, and is directed more particularly to recording and reading techniques for computer data storage DAT drives that avoids excessive contact between the read and record heads and the tape.
Digital audio tape (DAT) technology, which was developed for audio programming, has been adapted for storage of computer data. An example of a format for the storage of computer data utilizing DAT technology is the American National Standards Institute (ANSI) Digital Data Storage (DDS) standard, presently in draft form ("PROPOSED AMERICAN NATIONAL STANDARD HELICAL-SCAN DIGITAL COMPUTER TAPE CARTRIDGE 381 mm (0.150 in) FOR INFORMATION INTERCHANGE," ASC X3 Project No. 668-D).
The characteristics that have made DAT technology attractive for computer data storage include its high capacity, high transfer rate capability, relatively small media size and low media cost, and the adaptability of the DAT technology to conform with personal computer storage device form factors including the 5-1/4 and 3-1/2 inch forms.
As is well known, a DAT drive utilizes helical scanning pursuant to which a plurality of read and record heads are positioned appropriately on a cylinder that is tilted at a small angle relative to the travel path of the tape which is wrapped around a portion of the circumference of the cylinder. In operation, the cylinder is rotated at a relatively high rate while the tape is moved a relatively low rate. Ultimately, as a result of the relative motion of the recording heads and the tape, the read and record heads traverse tracks that are slanted or oblique relative to the longitudinal direction of the tape. The appropriate heads record and read such slanted tracks.
For digital data storage applications of DAT technology, for example, pursuant to the ANSI DDS standard, data is commonly stored in data groups respectively comprising 23 or 22 frames, where each frame includes 2 tracks. Typically, a data group includes about 126 Kbytes of information, as well as management information such as error correction information. In order to avoid the inertia and mechanical wear in starting and stopping the mechanical elements of a DAT drive and to increase throughput and for purposes of error correction, data is commonly written to and read from the tape in groups (i.e., a one group minimum) with the use of appropriate buffer management techniques.
A straightforward procedure for recording data with a computer storage DAT drive would be to record a complete group or groups to the extent available for recording without stopping the tape, and then repositioning the tape to be ready for the next group. Tape repositioning is necessary due to the tape drive mechanism inertia that prevents stopping the tape instantaneously. While the tape is being held in position ready for writing, the head cylinder continues to rotate, the tape remains tensioned and wrapped against the head cylinder, and the pinch roller remains engaged. A consideration with such procedure, however, is the continued abrasion of the heads against an area of the tape that was just recorded, and also tape damage from the pinch roller being engaged while the tape is stationary.
A straightforward procedure for reading data with a computer storage DAT drive would be to read a complete group or groups as can be buffered without stopping the tape, and then reposition the tape to be ready for the next group. While the tape is being held in position ready for reading, the head cylinder continues to rotate, the tape remains tensioned and wrapped against the head drum, and the pinch roller remains engaged. A consideration with such procedure, however, is the continued abrasion of the heads against the tensioned tape, and potential tape damage due to the pinch roller being engaged while the tape is stationary.
Techniques have been developed for retracting the tape from the head cylinder (commonly called "unloading" the tape) after a predetermined interval during which the tape has been kept stationary. Considerations with such technique, however, include the possibility of power being turned off while the tape is in the unload mode, which would prevent the recording of any partial data group that might have been buffered and also prevent the recording of an appropriate End of Data marker. In addition to being incomplete, such tape would be incorrectly formatted. Another consideration is that during the predetermined interval the tape continues to be tensioned against the rotating head cylinder, and the pinch roller remains engaged which presents the possibility of tape creasing.